For example, PTL1 discloses an air-fuel ratio detection device for an internal combustion engine that uses an in-cylinder pressure sensor to detect the air-fuel ratio or an air-fuel ratio imbalance between cylinders. The air-fuel ratio detection device changes the fuel injection amount for a target cylinder so that the burning velocity reaches a peak value. Information on the air-fuel ratio of the target cylinder is calculated based on the change of the fuel injection amount between the time when the fuel injection amount starts being changed and the time when the peak value of the burning velocity is reached.